There are many environmental challenges associated with the production of power by combustion. The mere acts of mining and transporting coal to the coal-fired power plants results in the generation of tons of coal fines (fugitive particles of coal dust). For the most part, these fines are not directly usable, and therefore large quantities of material are wasted and represent a environmental hazard and expensive disposal problem. Typically, coal fines are disposed of at or near the mine site in unsightly piles, trenches, or ponds. Currently, there are over two billion tons of discarded coal fines throughout the United States. While a portion of coal fines can be combined with coarser fractions of mine production for sale, the inclusion of fines often reduces the quality of the product below market requirements. Accordingly, coal fines handling, storage and disposal operations represent a significant and unproductive expense for the industry.
One approach to addressing the problem of coal waste is to form the fines into briquettes, which can be transported to power plants easily and once there, utilized efficiently. In the recent past, briquetting was thought to be the most desirable way to handle coal fines. Regrettably, power plants that used briquetted coal fines have had many handling problems associated with attempts to burn these products. These problems have been attributed to the methods of briquet manufacturing.
Generally, briquets are formed in two ways; either with a large amount of hydrocarbon or inorganic binder. Typically, in the case of hydrocarbon binders, asphalt cement or asphalt emulsions are mixed with the waste coal fines at levels above 5 percent by weight of the coal fines and then compressed into pellets or briquets. Power plants that utilize these briquets find buildup due to sticking of asphalt and coal fines on coal conveying equipment. Sticking in the bottom cone portion of the silo is a particular problem because it reduces fuel flow from the silo, which results in additional maintenance and reduced fuel flow. From the silo, the coal is passed through a size reduction mill to produce coal dust, which is then typically pneumatically conveyed to the burner nozzle. Because of the increased temperature in the mill, the asphalt becomes sticky, and briquets that are bound with hydrocarbon take on a taffy consistency rather than being reduced to powder. The result is reduced fuel flow through the mill and less fuel reaching the burner.
A second way to briquet is to use an inorganic binder, such as lime (calcium oxide or calcium hydroxide) or portland cement. These inorganic binders are normally added at concentrations of about 5 percent to 10 percent by weight of the coal fines. One problem with these binders is that they significantly reduce the heating value of the coal and increase the ash of the coal. This increases the loading on the pollution control equipment resulting in the increased risk of exceeding air pollution limits. Additionally, the ash fusion temperature of the coal is significantly reduced leading to a tendency to form slag around the burner. The production of slag in this manner increases burner maintenance and, in severe cases, leads to the burner being shut down completely so the slag can be removed. Finally, the practice of adding lime and cement binders in a dry state to coal can result in a exothermic reaction, which causes the coal to ignite after the briquets are placed in storage. Such storage pile fires are a safety and environmental concern as well as a waste of fuel material. Due to power plant burner fouling and transportation difficulties, briquetted coal fines are now considered a less desirable alternative fuel for power plants.
In spite of the issues surrounding the use of coal fine briquettes, recent changes in the law provide incentives for converting coal waste into synthetic fuel. To encourage the use of other fuels and to encourage the cleanup of fugitive coal fines and other high BTU matter that can be used as fuel, Section 29 of the IRS Tax Code provides tax credits for synthetic fuels produced from coal, municipal waste or biomass in a synthetic fuel plant. A significant tax credit is given to synthetic fuel plants based on the amount of synthetic fuel they utilize and its heating value. The code provisions were enacted to provide incentives to recover waste coal fines currently stored in holding ponds around the country, to recover the heating value from the voluminous amounts of municipal waste generated annually, to provide an incentive to substitute biomass for coal, petroleum and natural gas during the generation of electricity, and to reduce reliance on foreign fuel sources. Synthetic fuel plants that qualify for this tax credit can produce fuel for lower prices. Power plants can then purchase this inexpensive synthetic fuel and thereby not utilize natural resources and have an incentive to substitute coal, biomass, or municipal waste for imported petroleum and natural gas.
Synthetic fuel is combustible material that has undergone “chemical change.” This chemical change is generally determined utilizing chemical analysis equipment. Infrared spectroscopy (FTIR) is the method of choice for identifying changes in the molecular bonding or organic matrices such as those of combustible. In simple terms, absorption of infrared radiation occurs when the frequency of vibration of two atoms that are bound together by covalent or hydrogen bonding corresponds to the frequency of the radiation with which the sample is irradiated. The frequency at which a pair of bonded atoms oscillate is governed primarily by the identity of the atoms and, to a lesser extent, by their bonding environment, i.e., neighboring atoms or groups to which they are attached. Thus, an infrared spectrum can provide precise qualitative and semi-quantitative information on the nature of the molecular bonding within a given sample. Further, since infrared radiation is absorbed only by molecular bonds as opposed to individual atoms, changes in such absorption can be attributed to alterations in the molecular structure. This method is particularly sensitive to absorption by organic components and is useful for many inorganic components, though, in general, the sensitivity is not as great for the latter.
Of course, utilizing synthetic fuel and obtaining a tax credit cannot be counterproductive for power plants, or the plants will not be motivated to take steps to seek the tax credit. Therefore, obtaining and utilizing synthetic fuel is just the beginning of a power plant's fiscal concerns. In order to run efficiently, the power plant coal burner must utilize coal crushed to a uniform size and maintain a constant temperature. If the coal burner temperature is too low, slag will form in the burner. Slag periodically needs to be cleaned out of the burner causing the burner to be shut down during the cleaning procedure. The more slag that is produced, the more down time a coal burner will have. The ultimate goal in coal-fired power plants is to maintain a constant throughput of coal while maintaining a constant temperature, thereby producing power in the most efficient manner. Inefficient burning or down time because of increased slag causes the coal plant operators to utilize more natural resources in the form of coal to produce energy than would be necessary if the coal-fired plant was burning coal efficiently.
Of course, even when burning efficiently, coal-producing power plants are notorious for the environmental pollutants they produce. The burning of coal produces Priority Air Pollutants. These compounds include particulate matter, NOx, and SOx. Typically, most of these compounds are reduced from the stack emissions of the coal power plant by downstream and upstream environmental techniques. These techniques include the use of baghouses to trap particulate matter or scrubbers to trap SOx, NOx. Upstream techniques include the desulfurization of coal or using low-sulfur content coal as a fuel source.
Along with utilizing synthetic fuel to gain the direct economic benefit of a tax credit, it is certainly a goal of power plants to increase efficiency by reducing burner down time and to decrease costs associated with pollutant emissions. Generally, through a type of market control program under the Clean Air Act, power plants pay to emit pollutants. Typically, pollution credits are purchased yearly at a market price. If the owner does not use their credits, it can then sell them, usually for a profit. This type of market control makes it economically beneficial for power plants to reduce emissions.
Lastly, of extreme importance to power plants, is the BTU value of the fuel. This is the amount of energy that can be generated upon combustion. If the incoming fuel is too low in BTU value, the burner's throughput will be increased proportionally and burner down time will be more frequent. This concern, along with lowering emissions, and decreasing down time, creates a challenge to provide synthetic fuel for power plants. Certainly, it is in the best interest of power plants to utilize synfuel in order to obtain the immediate benefit of discounted fuel costs. If the synfuel also increased efficiency by lowering down time and burning to complete combustion while also lowering the production of priority air pollutants, the cost of producing power would decrease substantially.
Currently, a limited number of materials are being used for synthetic fuel production, none of which are completely effective. Examples include asphalt or asphalt emulsions, latex chemicals, and a proprietary polymeric material. Asphalt has been a more commonly used additive and provides a chemical change in the fuel product via the formation of hydrogen bonds between the asphalt and coal particles. However, this material suffers from several drawbacks: (1) the requirement that a much as 5 percent by-weight must be added in order to induce a consistent measurable change, so it is a costly additive; (2) the required chemical interaction does not occur with all coals, so it cannot be relied upon; and (3) the end-users, generally utility companies, encounter difficulties with crushing the synthetic fuel due to the high level of asphalt, which tends to clog the milling equipment, as discussed above, causing the fuel flow to decrease thereby reducing energy output. Market forces driven by the latter disadvantage results in a substantial discount in price for the sale of synthetic fuels produced with this level of asphalt. The high cost of this level of asphalt addition is also a major expense in the synthetic fuel production.
The second additive, polymeric precursors, suffers from an inability to consistently induce the prerequisite chemical change. The cost of polymeric precursors is a significant economic deterrent.
The prior art in the field of fuel additives for power plants has focused primarily on binding coal fines into strong, high BTU briquettes. Polymeric precursors and asphalt were often selected as binders because they have excellent binding characteristics and do not lower BTU value of the fuel. Because binding the coal particles together was the goal of this technology, the focus has been on providing a strong briquette with a high BTU value. These two parameters often necessitated the use of organic compounds because of there high BTU C—H bonds. However, the drawback of using organic compounds have been discussed above. Moreover, the organic compounds must be used in high amounts to bind coal, and at these high levels produce significant process handling problems for the power plant due to sticking buildup and fuel flow problems.
Much of the prior art uses varying levels of inorganic and organic compounds to form briquettes. For example, UK Patent GB 2181449 by Billcliffe et al. discloses the use of carbon dioxide, and either calcium oxide or calcium hydroxide at high levels in combination with a combustible material such as coal. U.S. Pat. No. 4,219,519 to Goksel discloses the use of calcium oxide or calcium hydroxide and silica to form briquettes from carbonaceous fines. Adding lime, limestone or dolomite and fly ash to finely divided coal as a binder to form durable pellets and agglomerates from finely divided coal is disclosed in U.S. Pat. No. 4,230,460 to Moss. U.S. Pat. No. 4,863,485 to Shaffer describes the use of polyvinyl alcohol and calcium oxide or magnesium oxide and water to form briquettes out of fine coal. U.S. Pat. No. 5,264,007 to Lask discloses the use, by way of example, of a lime and finely divided coke pitch to bind coal.
Each of these approaches employs high levels of inorganic lime, calcium hydroxide, or magnesium oxide. It is clear that the use of high levels of these compounds in fuel lowers ash fusion temperatures. The lower ash fusion temperature results in slag build up that ultimately requires the more frequent fuel burner maintenance and, in extreme cases, can result in such a large buildup that the burner needs to be shut down for cleaning. This can result in a utility not meeting its electric demand requiring the purchase of electricity from other utilities. This is an expensive risk for power plants when one considers that during these days of utility deregulation the power plant operator will be forced to purchase power for its customers at high market rates. Moreover, the cost of additives are prohibitively expensive. Additionally, the high lime concentration reduces heating value and the resulting ash increases the loading on air pollution equipment. In the first instance the use of high levels of inorganic compounds in the synthetic fuel causes burners to be taken off line more frequently. In a second instance, the use of expensive inorganics and organics as binders that do not reduce fusion temperature is cost prohibitive.
U.S. Pat. No. 6,013,116 to Major et al. is directed towards inducing a chemical alteration in synthetic fuel in order to qualify for IRS Section 29 tax credits. However, Major et al. is primarily focused on utilizing a binder for improved structural integrity in fuel briquettes or pellets. Further, this invention relies primarily upon lignosulfonate as a binder. Lignosulfonate is a relatively inexpensive waste product of the paper-making industry. It generally has a high BTU value but since it adds sulfur to the fuel, its use results in higher SOx emissions and the resulting need to purchase, rather than sell, priority air pollutant credits.
As the above has illustrated, the prior art utilizes additives at such high levels that the economic benefit of any foreseeable tax credit given for using synthetic fuel would be lost due to other inefficiencies and costs. As a result, the prior art does not solve the problem of providing a high BTU synthetic fuel that has consistently verifiable chemical change, thereby allowing the economic advantage of a tax credit while at the same time lowering pollution emissions without reducing power generation rate from the electric utility.